GENETIC IMPROVEMENT OF COWPEA (Vigna unguiculata (L.) WALP) FOR PHOSPHORUS USE EFFICIENCY
Cowpea is an important grain legume crop for millions of humans, fodder for livestock and income for all value chain actors. Its productivity is constrained by several biotic and abiotic stresses such as drought and poor soil fertility. Aligned with poor soil fertility in most growing areas, this dissertation describes phosphorus (P) use and acquisition of elite cowpea lines from different breeding programmes. In the first chapter, a brief overview was provided about cowpea as an important multi-purpose legume, constraints to its production and P deficiency and knowledge of farmers on using P based fertilizers. A participatory survey of farmers was conducted across 36 villages of major cowpea growing areas in northern Nigeria using a semi-structured questionnaire and focus group discussions to determine farmers’ perceptions on phosphorus fertilization, prevailing cowpea planting systems, use of improved varieties and farmers conceived production constraints and preferred traits. Results showed that farmers were aware of fertilizers as important for growth and increased yield but do not know major need of cowpeas for P. Intercropping with cereals was the most popular cropping system. Less than 6% used improved varieties of cowpea. Farmers identified insects and yield as the major constraint and preferred trait, respectively. Screening experiments, both in the screenhouse and low-P field, to identify and group elite cowpea lines based on performance under low-P and high soil P condition using shoot dry weight and other parameters was conducted. There was extensive diversity among the elite lines for adaptation to low-P and response to applied P fertilizer. A few cowpea lines such as IT97K-556-6, IT84S-2246-4 and IT89KD-288 produced above average yield under sub-optimal and high P conditions. The relative reduction in yield as a result of soil P deficiency compared to high P performance was over 50%. Cowpea lines with above average performance under the contrasting P soils were grouped as efficient and responsive lines and are suitable for cultivation under limited and optimum P input systems. There was a significant reduction in days to flowering and maturity among cowpea lines under high P conditions relative to low-P. To understand the genetics underlying P use and uptake efficiency, a quantitative trait loci (QTLs) mapping studies was undertaken through marker-trait analysis with a biparental recombinant inbred lines mapping population. A total of 16 QTLs were detected across seven of eleven linkage groups of cowpea. These QTLs were different from previously identified ones, indicating they are new QTLs under varying P conditions. These genomic regions were associated with flowering time, yield components and P use efficiency traits under low and high P conditions. In addition, a genome-wide association mapping study (GWAS) using DArTseq derived SNP markers on 400 diverse lines of cowpea to identify QTLs and SNP markers based on historic recombination events underlying the genetics of tolerance to low-P and response to applied mineral P fertilizer. GWAS mapping resulted in identification of over 60 SNP markers significantly associated with adaptation to low-P conditions and response to P fertilization as measured by differences in shoot dry weight, P use and uptake efficiency under two soil P rates. Summary of key findings of the research, conclusions and recommendations were made.